A practicable and accurate means for measuring ventricular volume in human beings is needed. The thermodilution method holds promise, but its accuracy is questionable. We have developed a model circulation system for studying indicator dilution methods. An animal or human left ventricle with valves attached is actuated and made to pump by applying programmed hydraulic pressure to its external surface. The temperature patterns that develop within the test ventricle following a cold injection are sufficiently repeatable to permit the use of several repeat runs to determine the temperature behavior throughout the cavity for any given set of conditions. A computer program digitizes and analyses the data and quantitates the mixing effectiveness of the ventricle. This mixing function is the main determinant of the accuracy of all indicator dilution methods. Objective 1 will be to conduct a study of mixing within the ventricle under several cardiac conditions. Parallel experiments will be made with the model system and on living dogs. The apparatus provides also for the injectate parameters to be varied conveniently. Objective 2 will be to determine optimum parameter values for making indicator injections under various cardiac conditions. It is conceivable that inadequacies of ventricular mixing can be compensated for by proper positioning of the temperature sensor or sensors within the ventricle and/or aorta and by proper selection of temperatures from the tracings. Objective 3 will be to find a method for obtaining an accurate mean temperature of ventricular contents at end diastole. After the first three objectives have been accomplished, the background information should be available for specifying equipment and procedures for an optimized thermodilution method for clinical use. Objective 4 will be to develop these specifications and to build prototypes of the required equipment. Objective 5 will be to test the procedure and equipment thoroughly both in vitro and in vivo and make a critical evaluation.